Our objectives are to elucidate the processes through which intracellular proteins are degraded. We wish to identify the rate-limiting steps involved, the location of the several steps in the pathways, and the mechanisms of regulation of protein turnover. Available evidence implicates the lysosomes as the locus of several steps in the process, including, in some cases at least, the rate-limiting reaction. In this connection we will continue to examine the capabilities and characteristics of lysosomes in the degradation of native proteins and the dynamics of turnover of lysosomes themselves. We have discovered a factor in rat liver which appears to mediate regulation of phosphofructokinase turnover. We propose to characterize this factor, determine the nature of the control of its level, and define its mode of action.